The Ultimate Guide to Heat Exchanger Tubes: Materials, Standards and Selection Criteria

Talk to any maintenance engineer about exchanger problems and, sooner or later, the tubes come up. Heat exchanger tubes are what actually move the heat, whether the unit is in a boiler house, a condenser bay, a refinery, or a chemical plant. If the grade, OD, wall thickness, or making process is wrong, the exchanger shows it soon enough. Usually it starts with poor performance, and then a retubing job arrives much earlier than planned.

Below, we will look at tube and exchanger types, the usual materials, ASTM specs, how tubes are made and tested, and a few things to settle before you place an order.

What Are Heat Exchanger Tubes and How Do They Work?

The idea is simple. It’s quite simple. There is a metal partition that separates the two fluids. Heat passes through the metal partition from the hot to the cold side, while the two fluids do not come in contact at any point in time.

For example, water at about 80°C enters the tubes and 30°C cooling water passes outside the tubes.

Of course, most plants don’t run under such comfortable conditions. The same tube could be carrying high-pressure steam one day and sitting in an acidic stream the next. It may also heat up and cool down every time the unit trips. So you can’t just choose a tube because it’s cheaper.

On heavy-duty jobs, a lot of engineers go for seamless heat exchanger tubes. There’s no longitudinal weld, so there’s one less spot where a crack or leak can begin. The exchanger design plays a part too. Fixed, U-bend, and tube-in-tube units don’t load the tube in the same way.

Types of Heat Exchanger Tubes and Tube Exchanger Designs

Each tube exchanger layout gains something and gives something up. The designer has to think about the temperature difference between the fluids, pressure on both sides, how much floor space is free, and how often someone will open the unit for cleaning. Then there’s thermal growth, because hot tubes get longer.

Fixed Tube Heat Exchanger

Here, the tubes are rolled or welded into a tube sheet at each end. Once the bundle goes in, it doesn’t come out.

The construction is simple. It works well, provided the shell and the tubes expand by about the same amount.

Inside the tubes, cleaning is not a problem. You remove the channel cover and run a brush or a hydro-jet lance down each tube. Outside is another matter. The bundle can’t be pulled, so scale on the shell side is hard to reach. For this reason, engineers check the temperature range carefully before they sign off on this design.

U Tube Heat Exchanger

In this type, every tube is bent into a hairpin, and both ends are fixed into one tube sheet.

Because the bent end is free, the tube can grow when it heats up without pulling on its joints. That’s why a stainless steel U tube for heat exchanger handles a wide temperature difference much better than a fixed one.

What’s the catch? The bend. Cleaning the straight legs is easy, but working a tool around that curve is slow, fiddly work.

Tube in Tube Heat Exchanger

Some people call it a double-pipe unit. A smaller tube runs inside a bigger one. One fluid goes through the inner tube, and the other travels through the ring-shaped gap between them.

It’s a plain design that suits smaller heat loads. Compared with a large shell-and-tube bundle, it’s usually quicker to open up for maintenance as well.

Materials Used for Heat Exchanger Tubes

The material to be used will be selected beforehand, and there will not be any room for change later on. There may come a time when the tube will leak despite its perfect size if it was made out of an unsuitable metal.

Carbon Steel

Carbon steel is the workhorse in plenty of plants. It’s strong, easy to buy, and reasonably priced, so it makes good sense when the fluid isn’t aggressive. Even then, the grade has to suit the design temperature, the pressure, the fluid, and whatever spec is written on the drawing.

Put it into a strongly corrosive stream, or push the temperature very high, and it won’t last long. At that point, most designers move to alloy or stainless grades.

Alloy Steel

Alloy steel has small amounts of metals like chromium or molybdenum added to it. These additions help the steel stay strong as the temperature climbs. You’ll see these tubes mostly in boilers, superheaters, and heat exchangers that run hot.

HMTL manufactures seamless carbon and alloy steel tubes for applications such as boilers, economizers, and heat exchangers.

For more details, check out hot-finished carbon alloy steel.

Stainless Steel

In cases where corrosion is the greatest concern, stainless steel would most likely be the material of choice. This metal is resistant to moisture, chemicals, higher temperatures, and frequent washings. However, there are grades of stainless steel that react differently, and this is where the mistake is made by buyers.

The case of chlorides can be cited here. A stainless grade that works effectively in freshwater may easily pit and crack in the presence of chlorides in the water. Check the fluid, the temperature, and the chloride content before you pick a grade. Ordering “any stainless” for corrosive duty happens more often than you’d think, and it’s an expensive habit.

For more details, check out stainless steel pipes.

Heat Exchanger Tube Standards and Specifications

What a spec number tells you is a lot more than what an alloy designation tells you. According to the specification in use, it dictates the composition, mechanical properties, dimensions, heat treatment, manufacturing process, and tests to be performed. Place two pieces of tubing side by side, and they could look identical but adhere to totally different specifications.

Commonly used ASTM heat exchanger tube standards include:

Standard	Typical application
ASTM A179/A179M	Seamless cold-drawn low-carbon steel heat exchanger and condenser tubes
ASTM A213/A213M	Seamless ferritic and austenitic alloy-steel boiler, superheater and heat exchanger tubes
ASTM A249/A249M	Welded austenitic steel boiler, superheater, heat exchanger and condenser tubes
ASTM B163	Seamless nickel and nickel-alloy condenser and heat exchanger tubes
ASTM A1016/A1016M	General requirements for specified alloy and stainless steel tubes

ASTM A179 specifies seamless cold-drawn low-carbon steel tubing for use in heat exchangers and condensers. It has the chemical composition requirements as well as several mechanical and testing methods. ASTM A213 is for seamless ferritic and austenitic alloy steel tubing. ASTM A249 covers welded austenitic steel tubing.

Whatever spec you’re buying to, check it against the equipment design, your purchase specification, and the real service conditions. Do this before the order is released, not after.

How to Select the Right Heat Exchanger Tubes

Do not begin with the price. Begin with the situation in which the tube will have to perform. A tube that is inexpensive but fails within two years does not remain inexpensive when considering downtime and labor losses.

1. Operating Temperature

First, determine the usual operating temperature. Then, determine how hot the tube would get during start-up, upsets, and cleaning cycles.

Metals usually become less strong as they heat up. You need a grade that maintains its performance over years of operation. A good test certificate is not sufficient.

2. Operating Pressure

The wall has to hold the pressure inside the tube. In some designs, it also has to take shell-side pressure from outside. OD, wall thickness, material strength, and design conditions are therefore calculated as a set.

A tube that’s fine in a low-pressure cooler might be unsafe in a high-pressure feedwater heater, even if the two materials look almost identical on paper.

3. Fluid and Corrosion Conditions

It is more common than not that the fluid determines the life of the tubing. Cooling water, steam, process fluids, and hydrocarbons all corrode metals in different ways. The mere presence of chlorides can turn a simple task into a challenging one.

Before selecting the material, it is important to know what the tubing will come into contact with and at what temperatures.

4. Heat Transfer Requirements

The whole point of an exchanger is to move heat. How much heat gets across depends on OD, wall thickness, surface condition, the metal’s conductivity, and the bundle layout.

There’s a trade-off here. A thicker wall is stronger, but it puts more metal between the fluids, and heat transfer goes down.

5. Tube Dimensions

OD and wall thickness usually come straight from the exchanger design. On big units, length starts to matter a lot. You may also have to define tolerances, straightness, end preparation, and U-bend radii. Settle every one of these before the PO is issued.

6. Manufacturing Condition

Hot-finished seamless tubes are good for big diameters and thick walls. Cold-drawn tubes are more accurate in size and have a smoother surface. Which one you pick depends on a few things, like the tube size, the material, where it will be used, and what the specification asks for.

7. Testing and Documentation

Industrial orders normally list the tests and paperwork needed. That could mean material test certificates, chemical analysis, mechanical results, dimensional reports, and NDT records. Agree on all this at the enquiry stage. Adding tests after the tubes are made costs both time and money.

Heat Exchanger Tube Manufacturing and Quality Testing

Every seamless tube starts as a solid round billet. The billet is heated, pierced, and rolled into a hollow shell. After that, it’s worked further until it reaches the right OD, wall thickness, finish, and mechanical properties. With hot finishing, the forming happens while the steel is still hot, which makes larger sizes and thicker walls possible.

HMTL’s hot-finish facility uses cross-roll piercing, accu rolling, and stretch-reducing mill technology for manufacturing seamless carbon, alloy, and stainless steel tubular products.

Depending on the product and the order, testing can include:

• Chemical analysis
• Tensile testing
• Hardness testing
• Flattening testing
• Flaring testing
• Flange testing
• Hydrostatic testing
• Eddy-current testing
• Ultrasonic testing
• Dimensional inspection

Eddy-current and ultrasonic tests deserve a special mention. Both can pick up hidden defects, and neither one damages the tube.

Common Applications of Heat Exchanger Tubes

No two plants run under quite the same conditions. That’s the main reason tube selection deserves care.

Power Plants

A power station uses these tubes in boilers, heat exchangers, condensers, and economizers. The tubes deal with high heat, high pressure, changing loads, and very long running hours. Strength and reliability come first here.

Refineries and Petrochemical Plants

You’ll find heat exchangers at almost every step of a refinery. The tubes might carry hydrocarbons, steam, chemicals, or hot process streams. For that reason, corrosion resistance and behaviour at high temperature usually settle the material question.

Chemical Plants

Chemical units often handle harsh media, and the conditions can change between batches. Sometimes a small change in the fluid is all it takes to need a different material.

Boilers and Economizers

The function of boiler tubes is to move water or steam and to absorb heat energy from the combustion gases. The economizer tube picks up the remaining heat energy in the flue gases to heat the feed water. Both have high temperatures and pressures. Thus, the material quality should match the application requirements.

HMTL manufactures seamless carbon and alloy steel tubes for boilers, economizers, and heat exchanger applications.

For more information, read about boiler & heat exchanger tubes for industrial applications.

Condensers

A condenser pulls heat out of a vapour, which turns it back into liquid. There’s fluid on both sides of the tube, so it has to fight corrosion and still pass heat across efficiently.

Process Industries

Process industries use heat exchangers wherever something needs heating or cooling. On a small skid, a compact bundle might be enough. A large plant, by contrast, could be running multi-pass bundles with hundreds of tubes.

Why Choose Heavy Metal for Heat Exchanger Tubes?

Heavy Metal & Tubes produces seamless carbon, alloy, and stainless steel tubular products for industrial consumers.

The hot-finished plant of the company has cross-roll piercing, accu rolling, and stretch reducing mills. The equipment is designed for producing tubes used in boilers, economizers, heat exchangers, and other harsh conditions. The company also has cold-drawn seamless capacity. Thus, consumers can choose which process meets their requirements concerning size, material, precision, surface finish, and application.

For fabricators and plant consumers, this is important because a tube should not only meet a certain grade but also meet requirements in terms of its dimensions, properties, testing, and standards.

Conclusion

The heat exchanger tubes are small components; however, they play a crucial role in the performance of the unit. Good selection of tubes depends largely on the service conditions - temperature, pressure, fluid, corrosion possibilities, heat transfer requirements, dimensions, and standards required.

The kind of exchanger also matters. Fixed-tube, U-tube, and tube-in-tube heat exchangers have different methods of installation and maintenance. The process is much easier when the heat exchanger tubes are produced by a manufacturer that has appropriate production facilities.